Security hardening: remove command injection paths and tighten input/memory validation

[Zer0plusOne]

Summary

This PR addresses multiple security issues in the scanner implementation and decoy pipeline.

Security fixes included

1. Removed command-injection paths in ARP resolution

   • Replaced shell-based system() / popen() calls with safe parsing of /proc/net/arp.
   • Added structured MAC parsing without shell execution.

2. Validated target IPv4 input

   • Added inet_pton-based validation for:
     • direct CLI target input
     • targets loaded from --file
   • Invalid targets are rejected/skipped safely.

3. Hardened realloc handling in decoy write callback

   • Switched to temporary pointer pattern.
   • Abort callback cleanly on allocation failure.

4. Improved error handling in init_decoy_ips

   • Added checks for curl_easy_init, JSON parse result, and expected JSON fields.
   • Added consistent cleanup on all error paths.

5. Validated CIDR prefix bounds

   • Added guard for prefix range 0..32 before mask operations.

6. Removed API key from source code

   • Replaced compile-time key usage with environment-variable based lookup (SHODAN_API_KEY).
   • Updated README accordingly.

Impact

These changes reduce RCE risk from untrusted input, prevent memory-safety issues on allocation failures, and eliminate secret exposure in source code.

Notes

• Functional behavior is preserved for normal valid inputs.
• --shodan now requires SHODAN_API_KEY to be set in the environment.